Californian Congressman Doug LaMalfa and 14 of his House peers have written a letter to the Department of Homeland Security demanding to know why the federal agency is buying so many rounds of ammunition and whether the purchases are part of a deliberate attempt to restrict supply to the American people.

A caller on my local talk radio station asked a gun shop owner about the ammo shortage.
The owner stated that there's not a shortage of production. But Osambo's u.S. Customs agents are refusing to sign off on imported ammo. This could also relate to interstate shipping of domestic ammo also.
All it would take to prevent an ammo manufacturer from shipping, is a government bureaucrat to lose a Bill of Laden. Or just file it away until enough gun shops scream enough.
And even then there's no promise it will be delivered in bulk.